DE 10 2007 053 632 discloses a method for coaxial beam analysis in optical systems. A defined percentage of the (laser) beam is reflected back therein coaxially or at a small angle via an optical surface which is located perpendicularly relative to the main beam, in particular a protective glass, and separated from the main beam for the beam analysis using a beam splitter. In order to be able to carry out a separation at the image side of partial beams which are reflected at the upper side and the lower side of the optical element, the optical element may have a wedge angle. Owing to the separated measurement of the partial beams, contamination at one side of the optical element is to be diagnosed.
EP 1 354 664 A1 describes a device for monitoring an optical element, in particular a protective glass of a processing head for processing a workpiece, which device has a separate light source for coupling a light beam into the optical element at an optical surface facing away from the workpiece and a detector for detecting a light beam reflected in the region of the optical surface facing the workpiece. In this instance, the light beam is reflected at the optical surface facing the workpiece on contaminants, for example, on smoke or splashes.
EP 1488 882 A1 also describes a device for monitoring an optical element, in particular a protective glass, in which the monitoring is carried out by the detection of scattered light on the optical element and the detection of scattered light on an additional optical element as a reference. In addition, detection of scattered light in the intermediate space between the first and the second optical element is provided for in order to determine the degree of contamination and the type of contamination (splashes or smoke).
In order to monitor the degree of contamination of a protective glass, it is known from DE 299 03 385 U1 to use a sensor which is arranged laterally on the protective glass and which is insensitive to the laser radiation itself but is sensitive to infrared thermal radiation which is emitted by dirt particles on the protective glass.
DE 10 2007 030 398 A1 describes an apparatus and a method for monitoring the temperature and/or a temperature-dependent characteristic value of an optical element wherein measurement radiation is emitted onto the optical element, at least a portion of the measurement radiation which has passed through the optical element is detected and the temperature or the temperature-dependent characteristic value is monitored based on a predetermined relation to the intensity of the detected measurement radiation.
EP 1 643 281 A1 discloses an optical element for installation in a mount, which has on the peripheral face thereof at least one profiling (for example, a notch). The mount may have a resiliently supported clamping member which has a pressing face for applying pressure in a radial direction onto the profiling and which includes a temperature sensor for measuring the temperature of the optical element.
DE 20 2008 010 495 U1 discloses an optical element and an apparatus for monitoring the optical element. The optical element has a base member which transmits radiation and which has on a peripheral face at least one planar face, the planar face being connected to a plate-like member which is transmissive for measurement radiation and the face of the plate-like member facing away from the base member having a coating which is reflective or anti-reflective for the measurement radiation.
In the above-described documents, the monitoring of the optical element is intended inter alia to serve to detect temperature changes during the operation of the optical element, which act on the optical properties thereof. In particular, a displacement of the focal position (focal drift) is intended to be identified in this manner.
JP20000094173 describes a device for adjusting the focal position of a laser beam which compensates for fluctuations of the focal position (“hot lens effect”) brought about by the heating of a focusing lens by the focusing lens being appropriately displaced in an axial direction. On the focusing lens, a temperature sensor is laterally arranged which is intended to enable the temperature of the focusing lens to be measured in real time.
From JP 7051875 A, it is known to arrange a device for detecting a laser beam diameter in the beam path downstream of a focusing lens of a laser processing head. Based on the detected diameter, a distance between the focusing lens and a workpiece is intended to be adjusted in an optimum manner by a device for automatic focal position adjustment.
DE 196 30 607 C1 discloses an apparatus for monitoring the energy of a laser beam, in which a laser beam is directed through a window into a chamber in order to irradiate a substrate. By inclined positioning of the window with respect to the axis of the laser beam, a beam portion is coupled out from the laser beam and directed onto a detector.
DE 10 2007 039 878 A1 discloses an apparatus and a method for focal position stabilization in optical systems for high-power laser radiation, wherein the focal point is displaced by movable optical elements and a control system in the event of a laser-beam-induced focal position change in the opposite direction so that the focal point overall remains in the desired position. The information required for the correction can be calculated by the momentary power of the laser beam. In order to measure this, there may be arranged in the beam path of the optical system at an angle with respect to the optical axis a plane-parallel plate on which a small constant fraction of the laser beam is redirected onto an optical sensor.